1. Field
One embodiment of the invention relates to a polymer electrolyte medium and a direct methanol fuel cell.
2. Description of the Related Art
As a polymer electrolyte medium (proton conductive membrane) of a direct methanol fuel cell (DMFC), hitherto, a perfluoroalkyl sulfonic acid membrane has been known, for example, Nafion (trademark, manufactured by DuPont), which is a fluorine system ion exchange membrane.
However, a conventional proton conductive membrane has a carbon fluoride structure in the main chain of its polymer structure, and has an affinity for methanol, and therefore, it is dissolved in a fuel cell during use and deteriorates. Further, the proton conductive membrane deteriorates because of radical seeds and the electric field caused by electrode reaction during operation of the fuel cell. Due to the deterioration of the proton conductive membrane by such factors, crossover of methanol takes place, and the efficiency of use of methanol declines. In addition, the output of the fuel cell is lowered. Moreover, since Nafion (trademark) is expensive, the cost of the fuel cell is increased.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 2003-36864 discloses a solid polymer electrolyte membrane for a fuel cell composed of a copolymer of a fluorine carbide vinyl monomer and a hydrogen carbide vinyl monomer, including a main chain having a carbon fluoride structure and a hydrogen carbide side chain having a sulfonic group, with an alpha-methyl styrene group coupled to the side chain.
However, the solid polymer electrolyte membrane has a carbon fluoride structure in its main chain, and has an affinity for methanol. Therefore, it is dissolved in the fuel cell during use and deteriorates, and further deteriorates due to radical seeds and electric field caused by electrode reaction. As a result, crossover of methanol takes place, and the output of the fuel cell is lowered.